Influence of climate change on low flow conditions. Case study: Laborec River, eastern Slovakia

Abstract

The paper deals with the long-term and seasonal variability of low flows using the example of a mountain river. The study covers the Laborec River in the eastern part of Slovakia, and the main aim of the research is to identify and establish long-term fluctuations of low flows on this river. The analysis aims to indicate trends of low flows and seasonal variability of outflows based on various measures and research methods as well as the links between them. Basic data on daily flow and precipitation series were collected from 1980 to 2019. Low flow periods were identified in relation to the fitting of the threshold level method to the 70th and 95th percentile on the flow duration curve as a constant, multi-annual cut-off (Q70%, Q95%). The longest lasting flows were those below q70%, which were determined in the shallow cut-offs that occurred for most of the year, i.e. from June to December and in January. The greatest culmination of flows below q95% was in August and September. The range of minimal unit outflow is the smallest in the summer-autumn period and results from long periods without precipitation and with increased evapotranspiration. The highest range of unit outflow was recorded from December to April. Knowledge of low river flows should be one of the important elements of advanced planning, which in the future may help to reduce conflicts between water users during the peak demand period.

Introduction

Periods of low flow, which depend mainly on climatic conditions, are an integral part of the hydrological regime for many rivers (Smakhtin 2001). Climate change is increasing the frequency and intensity of droughts in many parts of the world, leading to a greater risk of water scarcity (Palmer et al. 2008; Chen and Olden 2017; Marx et al. 2018; Kubiak-Wójcicka and Bąk, 2018). In the event of long and intensive droughts, the flow disappears from riverbeds. This phenomenon is particularly observed in dry and semi-arid areas and in the Mediterranean climate zone (Tal et al. 2010; Stubbington et al. 2018). Research carried out in recent years indicates that the disappearance of river flows is an increasingly common phenomenon in the rivers of temperate European climate (Datry et al., 2018). It is mainly observed in short rivers which are not covered by systematic hydrological observations (Rutkowska et al. 2018; Solarczyk and Kubiak-Wójcicka 2019). A lack of flow in small watercourses may result in low flows or may also lead to temporary disappearance of the flow of recipient rivers (recycler). Future climate changes may lead to more intense hydrological cycle processes. According to the latest climate change scenarios described by the Intergovernmental Panel on the Fourth Assessment Report on Climate Change (IPCC 2013), air temperature will continue to rise with increased precipitation variability. The increase in air temperature will increase the rate of evapotranspiration and accelerate the water cycle. Increased evaporation in summer will most likely cause more frequent and more intense low flow conditions. Such changes increase the risk of extremely low flows and droughts, which are critical for water management (The Intergovernmental Panel on Climate Change (IPCC) 2013; Huang et al. 2013; Štefunková et al., 2013; Shrestha et al. 2019; Kubiak-Wójcicka and Machula 2020). The number and duration of zero-flow periods are expected to increase further over the next century due to climate change and land cover and increased water extraction (Larned et al. 2010). The ever-increasing demand for fresh water for consumption, industry and agriculture has led to changes in river functions and threatens the resilience of aquatic ecosystems (Sharma and Dutta 2020; Tan et al. 2018; Kubiak-Wójcicka et al., 2017). Many studies have been conducted in order to understand the variability of stream flows caused by climate change and human activities. The reduction of flows and the accompanying reduction of water levels not only threatens river traffic and water supply for community use and resource development but can have serious consequences for the transport of sediments and pollution (Nilsson and Renöfält 2008; Mijuskovic-Svetinovic and Maricic, 2008; Gibson and Cai 2017; Gierszewski et al. 2020; Szatten and Habel, 2020) and the distribution of nutrients necessary for good ecological status of waters and wetlands (Zalewski 2002; Peters et al. 2006; Bradford and Heinonen 2008; Dembowska and Kubiak-Wójcicka 2017; Kubiak-Wójcicka et al., 2017).

Low flows are of great importance for the functioning of many processes taking place in river ecosystems (Postel and Richter, 2003; Garbe et al. 2016). Low flow is a relative term, which suggests that river flows fall below a certain level of expectation: there is no single low flow characteristic that is suitable for all purposes (Riggs 1980). The literature has calculated low flows using various methodologies, including hydrological methods, hydraulic assessment methods and habitat simulation methods (Smakhtin 2001; Snelder et al. 2011; Rolls et al. 2012; Acreman et al. 2014; Woo and Thorne 2016; Sadri et al. 2016; Kim and Choi 2019; Sharma and Dutta 2019; Zhao et al. 2020).

The basic method of separating subcompensatory periods is the threshold level method (TLM), which is based on the assumption that the subcompensatory period is the period in which the flows are equal to or lower than the accepted threshold level (Yevjevich 1967; Tomaszewski 2012). The threshold level can be established in many ways. Statistical, hydrological and economic criteria may be the basis for selecting the method of determining the limit level. The low flow thresholds adopted in drought studies usually vary between the 70th and 95th percentiles (Zelenhasić and Salvai 1987; Stahl et al., 2010; Hisdal et al. 2004; Demetrová and Škoda, 2009; Corzo Perez et al. 2011; Hasan et al. 2021; Tomaszewski and Kubiak-Wójcicika 2021). The adoption of a fixed baseline facilitates the analysis of process continuity and is a very useful tool to support the assessment of water management facilities and facilities dependent on river flow variability (Tomaszewski 2014). The analysis of a low flow regime is very important not only for research but also for practical purposes. The results of such analyses are also highly useful in water management planning.

The main research goal is the identification and stabilization of multi-year fluctuations in the low flows of the Laborec River. The analysis aims to show trends in low flows and the variability of seasonal runoff in the care of various measures and methods of research and the connection between them. The proposed patterns of changes in low flows on the example of a mountain river may provide for the transfer of a number of different studies and simulations of models for other rivers in Slovakia.